Functional programming

Rustom Mody rustompmody at gmail.com
Mon Mar 3 15:38:33 CET 2014


On Monday, March 3, 2014 7:53:01 PM UTC+5:30, Chris Angelico wrote:
> On Tue, Mar 4, 2014 at 1:08 AM, Rustom Mody wrote:
> >> How do you know that [1,2] is a list that must contain nothing but
> >> integers? By extension, it's also a list that must contain positive
> >> integers less than three, so adding [5] violates that. And [] is a
> >> list that must contain nothing, ergo it can't be added to, although
> >> (since it contains nothing) it can be added to anything.
> > If 'integer-less-than-3' were a type then yes there would be this
> > problem. More generally, if types could overlap then automatic
> > type-inference is impossible

> First, does Haskell allow this?

> ? [1,2,'foo'] ++ [3,4,'bar']

> If not, then find some other form of the expression that has the same
> point, and substitute in for the below. And then: which of these is
> permitted?


Dunno what you mean/whats the 'point'


> ? [1,2] ++ [3,4,'bar']
> ? [1,2,'foo'] ++ [3,4]
> ? [] ++ [3,4,'bar']
> ? [1,2,'foo'] ++ []
> ? ([1,2,'foo'] ++ []) ++ [3,4,'bar']
> ? [1,2,'foo'] ++ ([] ++ [3,4,'bar'])

> If it's okay to have heterogeneous lists, 

Its not.  None of the above work

If you want the (semantic) equivalent of python's [1,2,'foo']
you need to make an explicit union Int and String and its that
*single* union type's elements that must go in.

In all cases its always a single type. And so
sum([1,2,[3])
is a syntax error unlike python where its a runtime error





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